The present invention relates to an image-signal processing apparatus for enhancing a specific frequency component of an image signal and correcting the edge of a reproduced image.
One of image-quality control is edge correction for enhancing the edge of an image. FIGS. 1A, 1B, and 1C are schematic charts of signal waveforms for explaining the principle of edge correction. FIG. 1A shows the waveform of a luminance signal that is an original image signal. FIG. 1B shows the waveform of an edge enhanced signal according to the quadratic differential waveform of the original image signal. The edge enhanced signal is a signal in which the polarity of the original image signal is inverted after quadratic differentiation. The edge enhanced signal waves greatly at the rising and falling of the luminance signal, or at the edge of the image. FIG. 1C shows the waveform of an image signal subjected to edge correction, which is generated by combining the original image signal of FIG. 1A and the edge enhanced signal of FIG. 1B. The waveform of the image signal subjected to edge enhancement is corrected so as to fall once, then rise, and return after exceeding a specified level, at the rising of the original image signal. Thus, the edge of the image is enhanced to improve the distinctness of the image.
FIG. 2 is a schematic block diagram of a conventional edge-enhanced-signal generation circuit for generating an edge enhanced signal. A frequency component of an inputted signal in a specific band (for example, around 1.5 MHz) is extracted by a band pass filter (BPF) 2. The extraction process tends to generate a noise pulse. A coring circuit 4 is provided to eliminate the noise The coring circuit 4 allows only pluses whose amplitude exceeds a specified threshold to pass through and eliminates pulses below it as noise. The pulses that have passed through the coring circuit 4 are multiplied by a specified gain in a gain circuit 6, where the quadratic differential waveform is provided with an amplitude responding to the sharpness of the rising and falling of the luminance signal. Briefly, the sharper the edge of the original image is, the higher the degree of the edge enhancement becomes. However, excessive edge enhancement makes the image unnatural. A clipping circuit 8 is provided to prevent it. When the amplitude of the quadratic differential waveform that is subjected to gain control by the gain circuit 6 exceeds a set upper limit and lower limit, the clipping circuit 8 clips the waveform at the upper limit and the lower limit.
In addition to the edge enhancement, the luminance signal is subjected to tone correction by nonlinear transformation according to the characteristics of the display and the luminance distribution of pixels that make up the image so that the reproduced image is given visually preferable gradation. Known tone correction includes gamma correction, in which a low-luminance portion is enhanced and a high-luminance portion is made inconspicuous. There are several methods of edge enhancement in connection with the gamma correction.
In a first method, an edge enhanced signal is combined with a luminance signal and is then subjected to gamma correction. In this method, the high-luminance side and the low-luminance side of the image signal are asymmetrical in the effect of the edge correction under the influence of the gamma correction. More specifically, even if the way of rising of the luminance signal is the same in the original image signal, edge enhancement becomes relatively low when the rising occurs in a high-luminance region, while edge enhancement becomes relatively high in a low-luminance region. This poses the problem that the difference in the degree of enhancement depends on the gamma correction, resulting in a visually unnatural image.
In a second method, an edge enhanced signal is generated from an original luminance signal and combined with a luminance signal subjected to gamma correction. In this method, even if the way of rising of the luminance signal is the same in the original image signal, a change in the level of the luminance signal is relatively low when the rising occurs in a high-luminance region, while it is relatively high in a low-luminance region as a result of gamma correction. On the other hand, the edge enhanced signal is not influenced by the gamma correction to be at the same level irrespective of the luminance region where the signal rises. Briefly, this poses the problem that edge enhancement for the rising amount of the luminance signal is relatively high at a high-luminance region, while it is relatively low in a low-luminance region, also resulting in a visually unnatural image.
In a third method, an edge enhanced signal is generated from an image signal subjected to gamma correction. Accordingly, the level of noise pulses generated by differentiation of the BPF 2 varies depending on the level of the luminance signal. Specifically, the noise level is relatively low in the high-luminance region, while it is relatively high on the low-luminance region. This poses the problem that noise cannot be eliminated correctly by the coring circuit 4 that has a fixed threshold.
It is known in the art to provide a method described in JP-A-2003-32513 as a conventional method in which the problems of the above methods are solved. FIG. 3 is a schematic block diagram of a luminance-signal generation circuit for applying edge enhancement to a luminance signal by the conventional method. This circuit includes a main path for generating a gamma-corrected luminance signal from a image signal inputted by an image-pickup device etc. and an edge-enhanced-signal generating section 20 arranged in parallel with the main path, for generating an edge enhanced signal from the image signal. In the main path, a luminance signal Y is extracted by a low pass filter (Y-LPF) 22 and subjected to gamma correction by a Y-signal gamma correction circuit 24. In the edge-enhanced-signal generating section 20, an edge-signal generation circuit 26 with the same structure as that of the edge-enhanced-signal generation circuit shown in FIG. 2 extracts a specific frequency component corresponding to the edge to generate an edge signal. The edge signal is subjected to gamma correction different from the gamma correction by the main path to generate an edge enhanced signal. A combining circuit 28 combines the edge enhanced signal with the luminance signal outputted from the main path to output an output image signal subjected to edge correction.
The edge-enhanced-signal generating section 20 generates an edge enhanced signal as follows: The output signal from the edge-signal generation circuit 26 loses luminance information through the elimination of a direct current component, and so cannot be subjected to gamma correction in this state. Accordingly, an LPF (A-LPF 30) extracts a luminance signal from the image signal and then the luminance signal is combined with the edge signal outputted from the edge-signal generation circuit 26. The composite signal is subjected to gamma correction and so a signal in which the luminance signal subjected to edge-signal gamma correction is combined with an edge signal component subjected to edge-signal gamma correction is provided. Of the signal in which the two components are combined, an edge enhanced signal is a component arising from the edge signal. Accordingly, in combining the edge signal with the luminance signal, the edge-enhanced-signal generating section 20 generates two kinds of composite signals, a signal in which the edge signal and the luminance signal are added with an combining circuit 32 and a signal in which the edge signal is subtracted from the luminance signal with a subtracting circuit 34. The output of the combining circuit 32 is subjected to gamma correction by an edge-signal gamma correction circuit 36, while the output of the subtracting circuit 34 is subjected to gamma correction by an edge-signal gamma correction circuit 38. A subtracting circuit 40 subtracts the output of the edge-signal gamma correction circuit 38 from the output of the edge-signal gamma correction circuit 36, so that the luminance signal components in the outputs are cancelled, and so only a component arising from the edge signal is extracted as an edge enhanced signal.
With the conventional circuit of FIG. 3, the luminance signal and the edge signal can be gamma corrected on the basis of different transfer characteristic functions, facilitating the provision of a visually preferable reproduced image. On the other hand, the conventional circuit also has the problem that the structure of the edge-enhanced-signal generating section 20 is complicated and so the circuit becomes large in scale.